The present invention relates to photolithographic masks used in the production of integrated circuits. More specifically, the present invention relates to the production of attenuated phase-shift masks used for LSI and VLSI manufacturing, and in particular to the reticle alignment marks placed thereon.
Phase-shifting masks (PSM's) are distinguished from conventional photolithographic masks by the employment of selectively placed mask pattern materials allowing the transmission of exposure light with a phase-shift of .pi. (180.degree.). First pioneered in the early 1980's, this technique holds great promise for extending the limits of conventional photolithography to the production of circuit features as small as 0.25 .mu.m, and perhaps smaller. The 180.degree. phase difference created at the mask pattern edges sets up an interference effect that significantly enhances edge contrast, resulting in higher resolution and greater depth of field as compared to the conventional binary intensity masks utilizing only an opaque mask pattern material. Advantageously, the technique can be employed utilizing conventional photolithographic stepper optics and resist techniques.
Numerous PSM techniques have been developed. These include alternating, subresolution, rim, and attenuated phase-shifting techniques. Of these, the attenuated and rim phase-shifting techniques are the most versatile since they can be applied to any arbitrary mask pattern. As reported in Lin, Burn J., The Attenuated Phase-Shining Mask, SOLID STATE TECHNOLOGY, January 1992, attenuated phase-shifting masks have notable advantages over rim phase-shifting masks. For example, they can provide greater depth of focus, require shorter exposures, and use less mask area. Moreover, as explained below, an attenuated phase-shifting mask pattern can be formed from a single layer of material deposited on a substrate.
In the rim phase-shifting technique, an opaque layer of material is used to define the circuit features and a separate transmissive shifter layer is positioned about the edges of the features. In attenuated phase-shifting masks, a single slightly transmissive (halftone) absorber providing a phase-shift of 180.degree. can take the place of the conventional opaque, e.g., chrome, layer of mask pattern material. Prior art FIG. 4 provides a visual comparison of the cross-section, electric field amplitude, and image intensity for the conventional binary intensity mask (BIM), the attenuated phase-shifting mask (APSM), and the rim phase-shifting mask (Rim PSM).
In addition to the pattern used to form circuit features on a wafer, a photolithographic mask used for circuit production will include in its kerf (the periphery of the mask outside of the pattern area) reticle alignment marks used to precisely align the mask with respect to the wafer in the exposure tool, e.g., the scanner or stepper. Such marks will take various forms depending on the equipment being used, but typically will comprise hatch marks or geometric shapes arranged in various locations, orientations and patterns (e.g., chevrons, squares, circles and X-shapes). The marks may also be in the form of diffraction gratings. See, e.g., Aoki et at. U.S. Pat. No. 5,194,744 (hereby incorporated by reference in its entirety).
Precision stage metrology systems are used to perform the alignment of the wafer and mask. Such systems employ various optical detecting devices to precisely determine the position of the marks relative to a predetermined reference location or a determined position of marks placed on the wafer. Typically, laser or other visible light is used to illuminate the marks, and photodetectors sense the position of the marks from light reflected, absorbed or scattered by the marks. For example, using a Nikon step and repeat stepper, an Hg lamp is used to illuminate the marks and photodectectors sense the position of the marks from light reflected by the marks. Using an SVGL (formerly Perkin Elmer) Micrascan scanning apparatus, an Argon laser (488 or 514 nm) is used to illuminate the marks from one side of the reticle and a photodector positioned on the opposite side of the reticle detects the position of the marks from light absorbed by the marks.
Heretofore, attenuated phase-shifting masks utilizing a halftone material to form the mask pattern (in lieu of the conventional opaque mask material) have included reticle alignment marks also formed of the halftone material. This stems from the conventional practice of forming the mask circuit pattern and reticle alignment marks simultaneously in the same process steps (e.g., layer deposition, exposure and etching). The composition and thickness of the halftone layer will vary depending on the wavelength of the electromagnetic radiation, e.g., light, utilized in the wafer exposure step. In particular, since halftone materials exhibit different levels of light transmittance and phase-shift with respect to different wavelengths of light, the halftone layer composition and thickness must be varied for a given exposure wavelength in order to achieve the desired 180.degree. phase-shift and a suitable attenuation level (e.g., providing a transmittance of between 2-10%). As explained hereinbelow, applicant recognized that adjustment of the halftone layer to optimize phase-shift and attenuation for certain wavelengths of exposure light can lead to difficulties in achieving accurate and reliable mask/wafer alignment.